Wheelchairs often have a fixed seating surface that is either horizontal or slightly tilted back (i.e., the front edge of the seating surface is slightly higher than the rear edge of that surface). If a person sits in the same position in a wheelchair for a long period of time, pressure is continuously applied to the tissue on the buttocks, legs, and/or back that is bearing the person's weight in that position. Blood circulation to that tissue will be reduced, and ulcers or other problems can result.
To avoid these problems, it is necessary for people sitting in wheelchairs to shift their body weight from time to time. One way to accomplish this is for a nurse or attendant to manually tilt the entire wheelchair, or the seat portion of the wheelchair, backwards, so that the occupant's weight is shifted and the pressure point on the occupant's body is moved. However, it is desirable for the person in the wheelchair to be able to make this shift in position on her own, without assistance from an attendant.
To accomplish this, wheelchairs are sometimes provided with a motor-driven tilting apparatus. The occupant of the wheelchair can activate a switch or other control mechanism on the wheelchair, causing a motor to tilt the seat of the wheelchair, while the wheels and supporting frame stay in the same position. However, tilting the wheelchair seat in this manner also shifts the center of gravity of the occupant toward the rear of the wheelchair. The further back the center of gravity moves, the easier it is for the wheelchair and its occupant to tip over backward. This risk of injury to the occupant from tipping over is a serious problem, since the occupant of the wheelchair will typically have some physical disability that will make it difficult for her to break a fall.
To address this problem, wheelchairs are sometimes provided with a moveable pivot point upon which the wheelchair seat is mounted. A linear actuator is provided to raise the front end of the seat and tilt the seat back. However, the actuator demands a significant amount of vertical space. The seat of the wheelchair must be elevated to meet this demand. The wheelchair occupant typically disfavors the increased elevation.
There is a long-standing need for a wheelchair that allows the occupant to tilt the wheelchair's seat back while keeping the center of gravity as close as possible to the midpoint between the front and back axles, but that allows the elevation of the seat to remain unaffected.